Jack Sock, a 23-year-old American professional tennis player, was in the middle of a tournament match in Perth, Australia, when he stopped play to tell his opponent to challenge the umpire's call—not for his own benefit but his opponent's (video above).

Sock was playing against Lleyton Hewitt, once ranked #1 in the world, who looked at him in wonder at the level of sportsmanship rarely seen these days. 

Down 30-0 in the 2nd set, Sock needed the edge, but when the umpire called Hewitt’s first serve out, Sock stepped in to advise his opponent on the mistaken call.

"That was in if you want to challenge it," Sock said.

Hewitt, looking confused, took him up on it, and indeed the replay showed it was in-bounds and the point was awarded.

Sock went on to lose the match, but as the Youtube video's description reads, "Winning is important, but there is more to being a legend than win/loss records."